1. Field of the Invention
This invention relates to an improved process for the purification of phthalic anhydride produced by air oxidation of naphthalene. More specifically, the undesirable byproduct naphthoquinone is effectively removed from crude phthalic anhydride by treating the crude phthalic anhydride with small quantities of polymeric oils containing several double bonds optionally in the presence of alkali. By this process, phthalic anhydride of higher purity is obtained in higher yields than previously achieved.
2. Description of the Prior Art
Crude phthalic anhydride, which is produced by catalytic air oxidation of naphthalene, contains more than 0.5% by weight of naphthoquinone and other byproducts which are easily separated by distillation, such as maleic anhydride, sulfur and the like. Because naphthoquinone is within the boiling range of phthalic anhydride, its presence diminishes the color and purity of the phthalic anhydride. Therefore, it must be removed prior to distillation. It is highly desireable that the naphthoquinone content in the refined crude product should amount to less than 5 ppm. After distillation the pure phthalic anhydride should have a color value, according to ASTM Standard D1209, of 10 APHA at the most and a naphthoquinone content of 1 ppm at the most. Generally, the removal of the naphthoquinone takes place through conversion to higher condensation products, which then remain behind as a residue after distillation of the crude phthalic anhydride. There are several catalysts that have been recommended for this condensation, but all of them have considerable disadvantages. For example, in U.S. Pat. No. 2,557,499 tin (II) chloride combined with sodium carbonate was suggested. U.S. Pat. No. 2,855,440 recommends a combination of sulfuric acid and boric acid, and U.S. Pat. No. 2,670,325 recommends alkali hydroxide. According to Ullmann's Encyclopaedie der Technischen Chemie, 3d Edition, Vol. 13, p. 72, concentrated sulfuric acid at a temperature of about 200.degree. C. is disclosed as being preferred.
It is known, in practice, that sulfuric acid corrodes the reactor walls under the reaction conditions as well as gives off acidic gases. Furthermore, the neutralization of the sulfuric acid in the crude phthalic anhydride by using calcium carbonate produces inorganic salts which have a highly adverse effect by virtue of their precipitation in the distillation stills and the accompanying inhibition of the distillation. Alkali catalysts, on the other hand, result in the generation of the expected high concentrations of carbon dioxide from the phthalic acid present and in turn, significantly reduce the yield of phthalic anhydride. Attempts to bind the naphthoquinone in the phthalic anhydride to unsaturated natural oils like coconut oil, tung oil, unsaturated natural fatty acids, and the like have been unsuccessful. Only linseed oil lowered the naphthoquinone content to about 5 ppm; however, only strongly colored phthalic anhydride was obtained following vacuum distillation.